A burglar sign is a marking supposedly left on or near a property by a burglar or their accomplice, signalling in code that the building is vulnerable or unoccupied.
The practice is considered to be an urban legend, with Snopes commenting that publicly creating such signs would be an unnecessary additional risk for the burglar, when they could instead simply note down details of addresses. Those wishing to communicate property information to an accomplice could do so more easily by other channels, without making the same information available to other criminals.[1]